Proteases may function in carcinogenesis by inactivating or activating regulators of the cell cycle, differentiation, programmed cell death, or other processes affecting cancer development and/or progression. Consistent with the model involving protease activity and tumor progression, certain protease inhibitors have been shown to be effective inhibitors of carcinogenesis both in vitro and in vivo.
Aminopeptidases (APs) are a group of widely distributed exopeptidases that catalyse the hydrolysis of amino acid residues from the amino-terminus of polypeptides and proteins. The enzymes are found in plant and animal tissue, in eukaryotes and prokaryotes, and in secreted and soluble forms. Biological functions of aminopeptidase include protein maturation, terminal degradation of proteins, hormone level regulation, and cell-cycle control.
The enzymes are implicated in a host of conditions and disorders including aging, cancers, cataracts, cystic fibrosis and leukemias. In eukaryotes, APs are associated with removal of the initiator methionine. In prokaryotes the methionine is removed by methionine aminopeptidase subsequent to removal of the N-formyl group from the initiator N-formyl methionine, facilitating subsequent modifications such as N-acetylation and N-myristoylation. In E. coli AP-A (pepA), the xerB gene product is required for stabilization of unstable plasmid multimers.
APs are also involved in the metabolism of secreted regulatory molecules, such as hormones and neurotransmitters, and modulation of cell-cell interactions. In mammalian cells and tissues, the enzymes are apparently required for terminal stages of protein degradation, and EGF-induced cell-cycle control; and may have a role in protein turnover and selective elimination of obsolete or defective proteins. Furthermore, the enzymes are implicated in the supply of amino acids and energy during starvation and/or differentiation, and degradation of transported exogenous peptides to amino acids for nutrition. As leukotriene A4 hydrolase may be an aminopeptidase, APs may further have a role in inflammation. Industrial uses of the enzymes include modification of amino termini in recombinantly expressed proteins. See A. Taylor (1993) TIBS 18: 1993:167–172.
A variety of aminopeptidase have been identified from a wide variety of tissues and organisms, including zinc aminopeptidase and aminopeptidase M from rat kidney membrane; arginine aminopeptidase from liver; aminopeptidase Nb from muscle; leucine aminopeptidase (LAP) from bovine and hog lens and kidney; aminopeptidase A (xerB gene product) from E. coli; yscl APE1/LAP4 and aminopeptidase A (pep4 gene product) from S. cerevisiae; LAP from aeromonas; dipeptidase from mouse ascites; methionine aminopeptidase from salmonella, E. coli, S. cerevisiae and hog liver; and D-amino acid aminopeptidase from ochrobactrum anthropi SCRC C1-38.
Of these aminopeptidase, the structure of bovine lens leucine aminopeptidase (b1LAP) is well characterized and consists of a homohexamer synthesized as a large precursor, each monomer containing two thirds of the protein in a major lobe and one third in a minor lobe. The minor lobe contains the N-terminal 150 residues. All putative active site residues, presumably also the inhibitor bestatin-binding site, are found in the C-terminal lobe and include Ala-333, Asn-330, Leu360, Asp332, Asp255, Glu-334, Lys250, Asp273, Met-454, Ala-451, Gly362, Thr-359, Met270, Lys262, Gly362 and Ile-421.
Many aminopeptidase are metalloenzymes, requiring divalent cations, with specificities for Zn2+ or Co2+; however, particular sites of certain aminopeptidases can readily utilize Mn2+ and Mg2+. Residues used to ligand Zn2+ include the His His Glu and Asp Glu Lys configurations. In addition to bestatin, boronic and phosphonic acids, α-methylleucine and isoamylthioamide are identified as competitive inhibitors for most aminopeptidases. See A. Taylor (1993) TIBS 18: 1993:167–172; Burley et al. (1992) J Mol. Biol. 224:113–140; Taylor et al. (1993) Biochemistry 32:784–790.
Aminopeptidases from various organisms and various tissues within an organism have high degrees of primary sequence homology, as indicated by immunological assays. Some enzymes also exhibit very similar kinetic profiles. Direct amino acids sequence comparison of b1LAP and aminopeptidase-A from E. coli shows 18, 44 and 35% identity for the amino- and carboxy-terminals, and the entire protein, respectively. The comparison shows 46, 66, and 60% identity for the respective regions. See Burley et al. (1992) J. Mol. Biol. 224:113–140.
Bovine lens leucine aminopeptidase (b1LAP), bovine kidney LAP, human lens and liver LAPs, hog, lens, kidney and intestine LAPs, proline-AP, E. coli AP-A, AP-I and the S. typhimurium pepA gene product have been categorized as belonging to the family of zinc aminopeptidases which utilize the residues Asp Glu Lys for zinc binding and the active site amino acid configuration described above for bovine LAP for substrate binding. This family, possibly also including Aeromonas LAP, is suggested to be distinguished from zinc proteases which utilize His His Glu in zinc binding and Arg in substrate binding. The Saccharomyces methionine-AP is characterized to contain two zinc finger like motifs in the amino-terminus and shares little homology with b1LAP. See A. Taylor (1993) TIBS 18: MAY 1993:167–171; Watt et al. (1989) J. Biol. Chem. 264:5480–5487.
Leucine aminopeptidase expression is regulated at the transcriptional level, evidenced by enhancement of both activity and mRNA upon removal of serum in in vitro aged and/or transforming lens epithelial cells. Furthermore, LAP mRNA and protein is induced by interferon γ in human ACHN renal carcinoma, A549 lung carcinoma, HS153 fibroblasts and A375 melanoma. Regulation by development and growth is also implicated. The E. coli pepN gene is transcriptionally regulated upon anaerobiosis and phosphate starvation. Membrane bound AP-N (CD13) is expressed in a lineage-restricted manner by subsets of normal and malignant cells, apparently through regulation by physically distinct promoters. Expression of the yeast yscI product APE1 is dependent upon the levels of yscA and PEP4 gene products. Synthesis of APE1 is sensitive to media glucose levels, and the activity of yeast aminopeptidase is sensitive to substitution of ammonia rather than peptone as the source of nitrogen. See Harris et al. (1992) J. Biol. Chem. 267:6865–6869; Jones et al. (1982) Genetics 102:665–677.
Accordingly, aminopeptidases are a major target for drug action and development. Therefore, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown aminopeptidases. The present invention advances the state of the art by providing a previously unidentified human aminopeptidase.